1. Field of the Invention
This invention relates to a monitoring device, and more particularly to an improvement of monitoring device for forming display pictures by using a cathode ray tube.
2. Description of the Prior art
Conventionally, a monitoring device using a cathode ray tube has electric shields on the lateral and rear sides of the cathode ray tube for the purpose of reducing unnecessary radiation, thereby maintaining the leakage of magnetic and electric fields to level below specific standard levels.
However, leakage of electric field cannot be completely avoided by mere shielding of the lateral and rear sides of the cathode ray tube.
In particular, in the leakage of electric field, there is much unnecessary radiation from a screen of the cathode ray tube where the screen is not shielded. For instance, an A.C. electric field in the form of fly-back back pulses is detected when measured by, for example, a tabular electrode D which is placed in front of the monitoring device M as shown in FIG. 1.
The leakage of electric field is preferably diminished, considering undesirable effects on human health caused by such electric field.
An effective countermeasure is to provide a conductive coating on the surface of the cathode ray tube, and where the coating is grounded so as to reduce the leakage of electric field from the front face of the cathode ray tube.
This countermeasure, however, requires a special transparent conductive paint, and involves a problem that it costs too much, in regard to practicalness when mass production of the monitoring devices is considered.
It has also been proposed to use a conductive filter at the front side of a CRT in place of the conductive coating. However, this countermeasure uses extra special parts, and therefore also involves problems in regard to practicalness.
Still another solution is to use an antenna electrode disposed in the monitoring device and capable of generating an A.C. electric field so that the electric field leaked is canceled at the source of such electric field.
This method also is impractical due to employment of high voltage applied to the above-mentioned electrode, which causes danger while in service.